The impeller in a centrifugal pump, turbine, or compressor can, as a result of either bearing wear or of becoming loose on its supporting axle, move axially out of alignment. The resulting misalignment can permit the rotating impeller to rub against the housing of said pump. If the driving power source continues to rotate said impeller the cumulative wear will quickly destroy both the impeller and the housing. A search has revealed no prior device or method for early detection of an impeller's displacement from its correct operational location.